First Report of a New Myxogastria (Stemonaria longa) Causing Rot Disease on Shiitake Logs (Lentinula edodes) in China

From the paper

Abstract

The shiitake mushroom, Lentinula edodes (Berk.) Pegler, is widely cultivated and managed throughout China, with some of the highest yields occurring in Henan Province. The shiitake mushroom is one of the most important edible mushrooms in China, and many large-scale farmers in Asia and other parts of the world also grow and trade the crop. From June 2013 to August 2016, a serious rot disease epidemic affected many mushroom farms in Sanmenxia city (Henan Province). The infected mycelia in cultivated bags of L. edodes became yellow, and yellow drops of secreted water condensed on the surface of the bags, making them soft and fragile. When the disease infected a factory or farm, the mycelia were rendered unable to generate fruiting bodies, or the generated fruiting bodies exhibited stunted growth and spread the disease to adjacent fruiting bodies. Specimens were collected from cultivated L. edodes from the affected province and examined. The morphology of specimens collected from artificial log-cultivated shiitake mushrooms was described as follows: sporocarp stalks massed or crowded, flexous, black, long cylindrical, 5 to 20 mm long. Hypothallus silvery and shiny. Stalks were black, hollow, indistinctly and longitudinally striate or homogeneous. Columella tapered, almost reaching the top of the sporocarp. Capillitium open, dark brown, lax, its threads rather thin, forked, with long free ends and prominently spinose tips. Spore masses were black; spores verrucose-reticulate, 7.5 to 9 µm diameter. Plasmodium was transparent; before the formation of a fruiting body, plasmodium changed from transparent to white. The elongation factor 1-alpha (EF1α) and 18S ribosomal RNA genes were amplified with primers SMEF-F196/SMEF-R960 and 18S-F/18S-R. Nucleotide alignment showed 99% sequence identity (EF1α) with Stemonaria longa (KF743859.1); 99% identity (18S rRNA) with S. longa (KF743871.1). Based on the morphological characteristics, life cycle, cultural traits observed, and the sequences of EF1α and 18S rRNA analyzed, the slime mold was identified as S. longa (Nannenga-Bremekamp et al. 1984). To confirm its pathogenicity, six repeat bags of L. edodes were inoculated for each of the different growth periods and sprayed with 1 ml of spore suspension (10⁶ spores/ml); another three bags were sprayed with sterilized water as the control. All experiments were performed in triplicate. All treatments were maintained at 25 to 27°C and 86 to 90% relative humidity in the same mushroom-growing room. The pathogen was consistently reisolated from the infected fruit bodies of L. edodes. To our knowledge, this is the first report of S. longa causing rot disease on cultivated L. edodes. Effective measures need to be adopted to control this pathogen in order to prevent great losses to farmers in the future.